hung like a barnacle

now we all know that barnacles have the highest penis to body size ratio... but this site, OZ reef, puts it best...

Barnacles can extend it's penis up to 30 times the length of its body. Barnacles are sessile and unable to move from their settled location. In order to overcome this motility problem, the barnacle can extend it penis to any near neighbour in order to impregnate them with it's sperm.

so since this is a marine bio page, we will say hung like a barnacle...